The present invention relates to a laminated packaging material comprising a core layer of paper or paperboard and a gas barrier layer of polyamide, including a condensation polymer of metaxylene diamine and adipic acid (Nylon-MXD6), applied on one side of the core layer together with at least one layer of heat sealable plastics by means of co-extrusion coating.
The present invention also relates to a method of manufacturing of the laminated packaging material according to the invention as well as to a packaging container which is produced from the laminated packaging material.
Within the packaging industry, it is well known to employ laminated packaging material of a single-use nature for the packaging and transport of liquid foods. Normally, such laminated packaging materials are built-up from a configurationally rigid but foldable core layer consisting, for example, of paper or paperboard in order to achieve good mechanical configurational stability. Liquid-tight coatings of plastic are applied on both sides of the core layer and effectively protect the core layer of liquid-absorbing fibres against penetration by moisture. These outer layers normally consist of thermoplastic, preferably polyethylene, which moreover imparts superior thermosealing properties to the packaging material, whereby the packaging material may be converted into finished packages possessing the desired geometric configuration.
Laminated packaging materials consisting solely of paper or paperboard and liquid-Ught plastic lack, however, tightness properties vis-a-vis gases, in particular oxygen gas. This is a major-drawback in the packing of many foods, whose shelf-life, flavour and nutrient content dramatically deteriorate in contact with oxygen gas. One example of such foods is fruit juices whose vitamin C content declines when they are exposed to oxygen gas. In order to provide packaging materials with a barrier against gases, in particular oxygen gas, it is known in the art to apply a layer possessing superior oxygen gas tightness properties, for example aluminium foil (xe2x80x98Alifoilxe2x80x99), EVOH (ethylene vinyl alcohol) or PVOH (polyvinyl alcohol), on that side of the core layer which is intended to be turned to face towards the interior of the finished package.
It is generally desirable to be able to produce packaging containers for so called xe2x80x9cextended shelf lifexe2x80x9d (ESL) at cooled storage, i.e. to achieve maintained Vitamin C content and quality of the packaged product after about 6 weeks storage at 8xc2x0 C., about 8 weeks storage at 7xc2x0 C. or about weeks storage at 4xc2x0 C.
However, the known gas barriers suffer from certain drawbacks. For example, in certain cases from the points of view of cost, the environment and recycling, it has been deemed appropriate to replace Alifoil as the gas barrier material in food packages. Barrier polymers such as EVOH and PVOH, respectively are highly sensitive to moisture and rapidly lose their barrier properties against oxygen gas when they are exposed to a damp environment. This, among other things, make it necessary to surround gas barrier layers of EVOH and PVOH with layers of another polymer, for instance polyethylene, which is impervious to moisture. Alternatively, EVOH and PVOH, respectively, may be combined with one or more known food approved polymers for forming a continuous, well-integrated layer possessing superior gas barrier properties which are also retained in a damp environment. However, the manufacture of packaging materials including gas barrier layers with EVOH and PVOH, respectively, entails high costs for both material and for the production of the requisite multilayer laminate, since such gas barrier layers must be surrounded by at least one, often two protective outer layers of plastic on each respective side of the laminate.
U.S. Pat. No. 4,777,088 discloses a packaging laminate for the production of juice packages consisting of a core layer of paper or paperboard with a gas barrier layer which is applied thereon and which includes a nylon (not disclosed in any detail), as well as a layer of an ionomer binder, namely Surlyn(copyright) applied on the gas barrier layer as an intermediate bonding layer to the innermost (towards the inside of a package) polyolefin layer.
EP 0 520 767 discloses a packaging laminate consisting of a core layer of paper or paperboard and a gas barrier layer including an amorphous polyamide (Selar PA 3426 from DuPont Corp., USA), and a binder layer applied therebetween.
The main drawback with these known packaging laminates is, however, that they do not have sufficiently good gas barrier properties for the purpose of ESL packaging at economical polymer layer thicknesses.
The condensation polyamide polymer of metaxylene diamine and adipic acid, carrying the name xe2x80x9cNylon-MXD6xe2x80x9d, is a semi-crystalline polyamide and has special properties as compared with other conventional. polyamides, such as, for example, high tensile and flexural strength and modulus, higher glass transition temperature, lower water absorption, as well as excellent gas barrier properties against, for example, oxygen gas.
U.S. Pat. No. 5,164,267 (abandoned), describes a laminated composite consisting of a substrate sheet based on a cellulosic material laminated with a multilayer film comprising at least one layer based on a polyamide resulting from the polycondensation of aliphatic dicarboxylic acid with xylenediamine, e.g. Nylon-MXD6, wherein the multilayer film is co-extrusion coated onto the substrate with a polyolefin layer being the contact layer to the substrate.
However, it is in fact not possible to employ a gas barrier layer consisting solely of Nylon-MXD6 into liquid food paperboard or carton packaging laminates, since this material provides a brittle layer which readily cracks, for example on forming and folding of the packaging material, and therefore gives a poor barrier towards gases and liquids. Moreover, the Nylon-MXD6 in the gas barrier layer also appears to affect thermosealing properties in the process of sealing of the packaging material into packaging containers negatively, which also results in packages which are less gas-tight.
JP-A-06305086 describes a laminate from a biaxially stretched polyamide film and a paper layer, in which the polyamide film comprises at least two polyamide layers, at least one of the layers containing Nylon-MXD6. The biaxially oriented film is laminated to a paper layer by means of dry lamination methods, employing adhesives, or by means of extrusion lamination. Such a biaxially film is thus pre-manufactured by means of a different process, such as for example film blowing, and then laminated to other layers.
The resulting laminate from JP-A-06305086 is quite different from the one of claim 1 of the present invention. In order to provide adhesion between the paper layer and the polyamide layer, an adhesive, such as urethane adhesive, acrylic adhesive and polyester adhesive, or an intermediate bonding layer must be employed, which in turn requires more and/or different materials in the laminate and thus results in higher production costs and higher environmental impact, from both working environment and nature resource management points of view. Furthermore, the adhesion between the paper layer and the polyamide layer will most likely be worse in such a laminate, since the surface of a pre-manufactured film will be oxidised and/or hardened and will not easily adhere to an extruded bonding layer. In particular, the process is more cumbersome and less cost-effective for producing such a laminate, since it requires an extra step for pre-manufacturing of biaxially oriented film.
It is therefore an object of the present invention to realise a novel, cost-effective, environment friendly and well-integrated, laminated packaging material of the type described by way of introduction, which possesses excellent gas barrier properties, in particular against oxygen gas, as well as good liquid barrier properties and good mechanical properties, such as flexibility and adhesion strength between layers.
A further object of the present invention is to realise a packaging material for producing packaging containers which are particularly well suited for the storage of fruit juices with extended shelf-life of about 6-10 weeks at cooled storage.
These objects are attained according to the present invention by a laminated packaging material comprising a core layer of paper or paperboard and a gas barrier layer of polyamide including a condensation polymer of metaxylene diamine and adipic acid (Nylon-MXD6) applied on one side of the core layer together with at least one layer of heat sealable plastics by means of co-extrusion coating, in which the gas barrier layer further includes a second crystalline or semi-crystalline polyamide.
By mixing Nylon-MXD6 with another crystalline or semi-crystalline polyamide, such as for example PA-6 or PA-6/66, properties can be customised whereby, for example, improved elongation at break and improved sealing properties may be attained. The elongation at break of Nylon-MXD6 is only about 2,3%, while for a standard PA-6 it is normally 400-600%. However, an excessively high quantity of PA-6 will result in poorer gas barrier properties, since this is reduced exponentially with the quantity of PA-6. Examples of usable polyamides in this context are polyamide-6 (PA-6), PA-66, PA-6/66 and blends thereof.
Preferably, according to the invention, the blend of Nylon-MXD6 and the second crystalline polyamide is an immiscible blend, i.e. a two-phase blend for which a DSC measurement indicates two separate melt points or intervals, i.e. shows two melt-peaks instead of only one, with the Nylon-MXD6 as matrix. Such immiscible blends have the advantage of further improved oxygen barrier properties as well as improved tensile strength.
In order to achieve optimal properties in respect of gas barrier properties, mechanical properties, sealing strength and bulging resistance, the proportion of Nylon-MXD6 which is included in the mixture in the gas barrier layer according to the present invention is more than 50 weight per cent and less than 100 weight per cent, preferably 60-90 weight per cent, and most preferably 70-80 weight per cent.
According to one preferred embodiment of the invention, the second polyamide is a xe2x80x9cnylon clay hybridxe2x80x9d (NCH), which is a molecule composite consisting of a crystalline polyamide, such as for example PA-6, PA-66, PA-6/66 or PA-12, as well as uniformly distributed silicate layers. The NCH is formed in the polymerisation process by dispersing a clay mineral in the monomer and polymerising, which creates a morphology of extremely fine and well dispersed silicate platelets in the nylon polymer. This, thus, results in an improved oxygen barrier and excellent mechanical properties. Such polyamides are described, for example, in the xe2x80x9cjournal of Applied Polymer Science, Vol. 49, 1259-1264 (1993)xe2x80x9d, and xe2x80x9cVol. 55, 119-123 (1995)xe2x80x9d. The advantage with PA-6 is its low cost, while NCH based on PA-6, PA-66 or PA-6/66 enjoys the advantage that it gives considerably better oxygen gas barrier properties than its respective base polymers. Moreover, NCH is a better moisture barrier than pure PA-6xe2x80x94approximately twice as good. An example of an NCH suitable for the blend of the present invention is based on PA-6 and commercially available from UBE Industries (Grade 1022 CM1).
By thus blending Nylon-MXD6 with NCH based on, for example, PA-6, optimal gas barrier properties as well as mechanical properties are obtained. The proportion of Nylon-MXD6 (which is comparatively costly) can be reduced without the gas barrier properties being lost to the same extent as in the employment of pure PA-6. At the same time, there will be obtained a mixture with considerably higher elongation at break and thereby greater resistance to crack formation in forming and folding, providing a uniform, gas tight barrier layer. A blend of 75 weight % Nylon-MXD6 and 25 weight % NCH-PA6 has an elongation at break of more than 200%.
Furthermore, by using an NCH as the second component of the Nylon-MXD6 blend, the effect of xe2x80x9cbulgingxe2x80x9d is reduced. By xe2x80x9cbulgingxe2x80x9d is meant the effect that the packaging container walls are bellying outwards from the vertical plane between the corners of the package. The increased resistance to bulging by the use of NCH is probably partly due to the contribution of stiffness properties from the NCH material itself. The tensile modulus of NCH-PA6 is for example about 830-880, while for PA-6 it is only about 580-600 N/mm2. In addition, the moisture barrier properties of NCH are approximately twice as good as the one of PA6. It is of great importance to reduce the effect of bulging, since the customers in some countries are particularly prejudiced towards the bulged appearance of packaging containers, believing the bulged appearance being due to the food products being fermented or the like.
According to a second preferred embodiment of the present invention, an even more cost-effective and environment friendly packaging laminate in which the gas barrier properties are further improved is provided, at the same time as a packaging laminate better adapted for the manufacturing of packaging containers having improved seals may be provided. These objects are achieved by applying the polyamide gas barrier layer by means of co-extrusion coating directly onto the core layer of paper or paperboard, without any interjacent adhesive or bonding polymer layer. In this way, intermediate bonding layers are made superfluous and material is saved, thus providing an economical laminate both from an environmental resource, recycling and cost point of view. By the term xe2x80x9cextrusion coatingxe2x80x9d is thus meant the simultaneous extrusion and application of a layer of extrudable plastics onto a substrate, which is different from so-called xe2x80x9cextrusion-laminationxe2x80x9d, i.e. the lamination of a pre-manufactured film to a substrate by means of extrusion of an intermediate bonding layer between a web of the substrate layer and the pre-manufactured film layer. It has been shown that the gas barrier property of a laminate having a three-layer structure with a nylon-MXD6 blend gas-barrier layer, a tie layer and a polyolefin layer coated on the inside of the core layer by co-extrusion, is improved by about 30-40%, compared to that of a laminate having a five-layer structure with a further polyolefin layer in contact with the paperboard layer and a tie layer between the polyolefin layer and the gas barrier layer.
Furthermore, when co-extruding a three-layer structure in one step onto the paperboard, the outermost polyolefin layer of the three layers may be extruded at a lower temperature than when co-extruding a five-layer structure having two outer polyolefin layers onto the paperboard. In five-layer co-extrusion by means of three extruders and a five-layer feed-block, which normally is the case (it is desirable to involve as few extruders as possible in a process), the two outer layers necessarily are extruded at the same temperature. In order to provide adhesion between the layer of LDPE and paperboard, a temperature of about 320xc2x0 C. would be desirable. However, for the outside layer of the laminate, the LDPE may be extruded at the much lower temperature of 280xc2x0 C. LDPE extruded at such a low temperature will be subject to less oxidation and will be more suitable for the subsequent heat sealing in the process of converting the packaging laminate into a paper container. The risk of obtaining a taste of xe2x80x9cplasticxe2x80x9d of the packaged product will also be eliminated if the polyethylene is extruded at the lower temperature. In a five-layer structure, an extrusion temperature somewhere in between must be chosen as a compromise, in order to balance the adhesion to the paperboard against the heat sealability of the outer LDPE layers, since the two LDPE layers originate from the same extruder.
A gas barrier layer comprising the blend of Nylon-MXD6 and PA-6 or NCH has proved to adhere very well to a core layer of paper or paperboard at such high line speeds as are necessary for the production of cost-effective laminates. This is not at all self-evident because different polyamides different properties in this respect. PA-6 normally adheres well to paper board, while an amorphous polyamide does not adhere. With good adhesion is meant that the plastic layer adheres to the paperboard with a strength greater than the cohesion within the paperboard itself. Thus the rupture appearing in a peeling test occurs within the paperboard layer and not between the layers. This can be seen in that the xe2x80x9cpeeledxe2x80x9d plastic layer surface is covered with paper fibres. Similarly, layers of NCH or of a blend of PA-6 and NCH do not adhere to paperboard, while Nylon-MXD6 adheres to a certain extent. However, the bonding between the Nylon-MXD6 and the paper will be easier destroyed because the layer of Nylon-MXD6 is brittle and inflexible in itself and will crack and delaminate from the paperboard when the laminate is bent or twisted.
In comparison with lamination of a gas barrier layer on the core layer with an interjacent layer of polyethylene, it has moreover proved that application of the gas barrier layer directly on the core layer resulted in an approximately 30-40 per cent improved oxygen gas barrier. This may be because of an equalisation of the moisture between the core layer and the gas barrier layer on direct application. When the gas barrier layer is in direct contact with the paper or paperboard layer, the result will, namely, be that moisture from the contents of the packaging container which penetrates into the gas barrier layer is distributed over both the core layer and the gas barrier layer. As a result, a smaller proportion of moisture will remain in the polyamide layer, for which reason the gas barrier properties are better retained in the gas barrier layer in this particular case.
This surprising increase of oxygen barrier properties does not generally apply to all polyamides. It is specific to Nylon-MXD6 only, and probably is due to the fact that the gas barrier properties of Nylon-MXD6 decrease at high relative humidity, as is usually the case at packaging of liquid food products.
The gas barrier layer may naturally be applied in any desirable thickness but according to a preferred embodiment of the present invention, which is particularly suitable for packaging containers intended for fruit juices with extended sheqlf-life, the gas barrier layer is applied on the core layer in a quantity of approx. 3-30 g/m2, more preferably 4-12 g/m2 and most preferably 5-8 g/m2. The reason for this is that, in application quantities below 5 g/m2, uncertainty in the process as regards application and barrier properties may be great. In application quantities above 8 g/m2, the packaging laminate becomes less cost-effective.
On that side of the gas barrier layer which is turned to face away from the core layer, a polyolefin layer may be applied, which is bonded to the gas barrier layer by means of a layer of adhesive polymer applied between the gas barrier layer and the polyolefin layer. The polyolefin layer may comprise different types of polyethylene, for example ultra low density polyethylene (ULDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and metallocene polyethylene (m-PE) or mixtures thereof. In particular, m-PE and mixtures of m-PE and certain of the above-disclosed polyethylene types give extremely tight seals on thermosealing of the packaging material into finished packaging containers, which also favours the gas tightness of the packaging container. The gas tightness of a packaging container thus depends both on the gas tightness of the packaging material per se and on how tight the seals are which it is possible to achieve in the production of packaging containers from the packaging material.
The layer of the abhesive polymer disposed between the gas barrier layer and the polyolefin layer consists, for example, of a polyolefin modified with carboxylic acid groups, for instance polyethylene grafted with maleic acid anhydride, such as certain types of Admer(copyright) and Bynel(copyright). Alternatively, mixtures of adhesive polymer and PE may be employed in order to provide adhesion to the polyamide gas barrier layer.
The purpose of the two outer polyolefin layers is, on the one hand, to protect the packaging material against penetration by moisture and liquid from the outside and, on the other hand, the key function of rendering the packaging material sealable by conventional so-called thermosealing whereby mutually facing layers of plastic, under the supply of heat and pressure, can be joined together by surface fusion. Thermosealing realises mechanically strong, liquid-tight sealing joints during conversion of the packaging material into packaging containers. In order to realise seals possessing good tightness, the inner polyolefin layer is applied in a quantity of approx. 15-35 g/m2, preferably approx. 25-30 g/m2 and the outer polyolefin layer in a quantity of approx. 12-20, preferably 15-20 g/m2. In the event of LDPE being in the inner layer, the quantity should be at least approx. 25 g/m2, preferably at least approx. 30 g/m2. The inner polyolefin layer 13 can also be applied as two or more separate polyolefin layers consisting of the same or different types of polyolefin in a quantity which in total amounts to the above-disclosed quantity.
The outer polyolefin layer, which is applied on the packaging material on that side of the core layer which, in the finished packaging container, is intended to be turned to face towards the outside, may be provided with suitable print of a decorative andlor informative nature for identifying a packed product.
According to a third preferred embodiment of the present invention, a packaging laminate having improved sealabilty properties in the process of converting into a packaging container is provided. It has been discovered that it is important to have certain minimum thicknesses surface weights of the two outer thermoplastic layers of the laminate in order to achieve optimally strong, gas-tight seals and reduced bulging. This is in particular apparent for a laminate with a three-layer co-extrusion coated barrier film. In such a three-layer laminate, less liquid barrier plastics are involved and the risk for bulging is increased. It has now been seen that, in order to increase the seal strength and reduce bulging the amount of sealing polymer in the innermost layer, i.e. the preferable low density polyethylene LDPE), on the inside of the packaging laminate should be at least 25 g/m2, when the polyamide barrier layer and the tie layer are applied in quantities of about 6 and about 3 g/m2 respectively, while the outermost layer of preferably LDPE should have a surface weight of about 15-20 g/m2. Expressed with other words, there should be a certain relationship between the total amount of liquid barrier polymer on the inside of the paperboard and the amount of polymer on the outside of the paperboard. Thanks to the improvement in seal strength and tightness of the sealed joints towards liquids and gases, a packaging container with improved retention of vitamin C in the packaged product is furthermore obtained. Less moisture can penetrate into the laminate itself via the seals, which results in the improved resistance to bulging of the container walls. By accordingly adjusting the amounts of outer polyolefin layers so that the amount of LDPE in the innermost layer is at least 25 g/m2, preferably 30 g/m2, and the amount in the outside layer is less than 20 g/m2, preferably about 16 g/m2, the barrier layer being applied by 5-8 g/m2 and the tie layer being applied by 3-6 g/m2, these improved properties can be secured. This effect can be seen also for five-layer laminates, the amounts however being less critical than for the three-layer laminate. Most surprisingly, however, a three-layer laminate has improved seal strength and bulging properties in comparison to a corresponding five-layer laminate, in which a much higher amount of liquid barrier polyolefin is applied on the inside of the core layer towards the product.
According to a fourth preferred embodiment of the present invention, a cost-effective packaging laminate having a further extended shelf life together with good aroma and flavour retention properties, so-called non-scalping properties, is provided.
The Nylon-MXD6 material in itself also has excellent xe2x80x9caroma barrierxe2x80x9d properties, i.e. barrier properties towards aroma and flavour substances, so-called non-scalping properties.
The gas barrier properties of the preferred laminate and the retention of vitamin C in the packaged product from the laminate are also further improved. These objects are achieved by a packaging laminate having a first barrier layer deposited on the inside of the paperboard substrate core layer and a second barrier layer, including a blend of Nylon-MXD6 and a second crystalline or semi-crystalline polyamide, deposited further towards the inside and the packaged product, and with relatively thin layers of tie and polyethylene product contact layer between the innermost barrier layer and the packaged product. Because of the lower amount of polyolefin polymers applied as innermost layers of the laminate, scalping of non-polar substances, such as certain flavour and aroma substances, from the product into the packaging material will be prevented. The aroma and flavour substances will migrate into the relatively thin product contact layer, but as they reach the barrier layer of the blend of Nylon-MXD6 and the second crystalline or semi-crystalline polyamide, they will be prevented from migrating further and, accordingly, less of said substances will be able to be absorbed into the polyolefin layers. Since the innermost polyethylene layer is rather thin in this preferred packaging laminate, it will not be sufficient for creating a sealing bond when converting and sealing the laminate into a packaging container. Therefore, further sealing layers of thermoplastic polymer are employed on the inner side of the core layer, on the other side of the innermost barrier layer, i.e. between the first and the second barrier layers. At sealing, the rather thin product contact layer and the innermost barrier layer will be xe2x80x9csealed throughxe2x80x9d, i.e. the polymers in those layers will be molten away and the heat will reach and melt also the intermediate layers of heat sealable polymers. In this way, more heat sealable polymer will be available for creating a heat seal, despite the thin inside outer layer of the laminate.
This preferred high performance ESL packaging laminate will of course also have the advantage of the gas barrier layer in direct contact with the paperboard, providing a 30-40% improved gas barrier due to the lower moisture content in the gas barrier layer.
According to a further aspect of the invention, a method of manufacturing of the packaging laminate according to the invention is provided as set out in claim 14.
The laminated packaging material of the invention is preferably manufactured by means of a one-step co-extrusion process, in which all the polymer layers on the inside of the core layer are applied on the core by means of co-extrusion. A heat sealable polymer may be applied onto the outside of the core layer, i.e. the side of the core layer which is turned to face away from the barrier layer, before or after said one-step co-extrusion process. One important advantage with co-extrusion is that the heat from the molten polymer will be better preserved in a multilayer extrusion film until the extrusion film hits the substrate onto which it is extrusion coated, thus providing for improved adhesion to the substrate (due to high so-called xe2x80x9cthermal inertiaxe2x80x9d). Another advantage is that, a further extrusion process step is saved, thus providing for a more time- and cost-effective process.
In order to obtain sufficient adhesion between the multilayer co-extruded film and the paperboard substrate, the surfaces should be activated by a pre-treatment such as treatment with corona and/or flame or ozone. Such surface activation treatment methods are well known in the art. Preferably, the paperboard substrate is pre-treated by means of flame treatment and/or corona treatment, of which flame treatment is the most preferred, while the freshly extruded film is preferably treated with ozone before it is coated onto the paperboard.
A configurationally stable packaging container produced from the laminated packaging material according to the present invention is disclosed in appended claim 16. The packaging container according to the invention may be provided with an opening device, such opening closing devices being known to the skilled person in the field of liquid food packaging.
Thanks to the present invention, there will be realised a more environmentally friendly, cost- and production-effective packaging material which displays excellent gas barrier properties, in particular against oxygen gas, even when exposed to a damp environment. The packaging material according to the present invention also possesses good internal cohesion in order to counteract delamination on use of the packaging material for the production of packaging containers intended for liquid foods, in particular fruit juices with so-called extended shelf-life, i.e. for cold storage for a period of up to 4-12 weeks.